Lysozyme is widely used in food and pharmaceutical treatment. Lysozyme is commonly applied in food preservation and processing, including aquatic products such as oysters, shrimp, and the like; fresh foods such as bean curds, vegetables, fish, fruits, and the like; and processed food such as sushi, cooked noodles, fish balls, meat balls, fish flakes, and the like. In addition, lysozyme is widely applied in the manufacture of cheese. Lysozyme has the same role as rennet which makes casein in the milk unstable to form curd.
Pure lysozyme is also used as an effective ingredient for preparing medicine. For example, lysozyme is used in a medicine for treating the wounds of chronic rhinitis or as a preserving agent for pharmaceutical products such as eye drops.
Chicken egg white containing about 3.5% lysozyme (based on dry weight) is the main material for preparing lysozyme. At present, the primary industrial method for preparing lysozyme is direct crystallization [Alderton, G. and Fevold, H. L. Direct crystallization of lysozyme from egg white and some crystalline salts of lysozyme. J. Biol. Chem. 104:1(1946)]. This method involves adding 5% sodium chloride to the chicken egg white liquid, adjusting the pH level to 9.5, adding a small amount of crystal seeds, and crystallizing at 4° C. for about 5 days. The recovery rate of lysozyme is about 60 to 80%. However, the recovery rate for this method may significantly vary depending on the treating amount of raw material. Chiang et al. [Chiang et al., Research on purifying lysozyme from chicken egg white by ultrafiltration, J. Chin. Agric. Chem. 24(1): 86(1986), Taipei] conducted an experiment in the same manner using 700 mL of chicken egg white liquid, and found that the recovery rate for a single crystallization was 43.0%, and the purification efficiency was 4.9 fold; the recovery rate after double crystallization dropped to 12.4%, and the purification efficiency is 7.9 fold; the recovery rate after triple crystallization was 11.8%, and the purification efficiency was 9.1 fold. The results show that the recovery rate was significantly reduced, and the purification efficiency was still not satisfactory after many operations. Furthermore, a great disadvantage was that the large amount of egg white liquid used in the process contained a high concentration of sodium chloride after the separation of lysozyme. Therefore, the recovered egg white liquid could not be effectively used any more after the separation. To address this problem, many researchers tried preparing lysozyme by ultrafiltration. The research of Chiang et al. (the same research as above) showed that the recovery rate for a single ultrafiltration accompanied with crystallization was 47%, and the purification efficiency was 3.0 fold. Chiang et al. disclosed that the lysozyme recovery rate after ultrafiltration by using a membrane with a molecular weight cut-off of 30,000 was 96%, and the purification efficiency was only 6 fold, indicating that the purification effect was not satisfactory [Chiang, B. H., et al. Egg White lysozyme purification by ultrafiltration and affinity chromatography, J. Food Sci. 58(2): 303(1993)]. Furthermore, the great disadvantage of the ultrafiltration method was that the filter membrane was easily blocked, and thus mass production becomes difficult.
In addition, there is a method for purifying lysozyme by using an adsorbent. Alderton et al. [Alderton, et al. Isolation of lysozyme from egg white, J. Biol. Chen. 157: 43(1945)] disclosed that bentonite has the property of adsorbing lysozyme, but was not easily eluted. It needs to be eluted with a solution containing 5% acetic acid in pyridine. As a result, this method was not suitable for industrial production.
Therefore, the present inventor have undertaken extensive studies in order to solve the above-mentioned problems and found that diatomaceous earth, kaolin, and zeolite could specifically adsorb the lysozyme, and the adsorbed lysozyme could be easily eluted with a salt solution. Furthermore, the unadsorbed egg white has the same processing properties as the original egg white. Meanwhile, the unadsorbed egg white liquid is not contaminated by any chemicals. Therefore, it can be directly used in food processing as the original egg white. The present invention has been accomplished based on the above finding.